Orthoclase: Gemstone Information
Orthoclase is a component of many rocks and is typically found in acidic and alkaline igneous rocks particularly in granite pegmatites and syenites. It also occurs in Alpine-type metamorphic rocks and hydrothermal veins – additionally as detrital grains in placers deposits. In Madagascar, the gemmy ‘orthoclase’ found in pegmatite veins associated with medium to coarse grained marble lenses has since been found by X-RD to be high to low sanidine.
Orthoclase is one of the most common minerals and is found worldwide; however, it is very rare to find orthoclase as transparent gem-quality crystals. Only a few notable localities are mentioned: The gem gravels of Myanmar and Sri Lanka Similar produce similar material. Much adularia is found in the Alps in Switzerland, especially at Adular, the locality this variety was named after. ‘Carlsbad Twins’ are famous from Carlsbad, Czechoslovakia and fine ‘Baveno’ twins have come from Baveno, Italy.
Large, well-formed crystals were found in Disentis, Switzerland. Lightblue crystals occur in the Lake Baikal area in Russia, and in Udacha, Russia, yellow-brown transparent orthoclase up to 8–10 cm is known. Large masses occur on Mt. Kilimanjaro, Tanzania. In the USA, enormous crystals were found on Sandia Mountain, Bernalillo Co, New Mexico. Famous collector specimens have come from Robinson, Colorado and Goodsprings, Clark Co., Nevada. Orthoclase has also come from the French Creek mine, St. Peters, Chester Co., Pennsylvania. Much adularia
moonstone has come from New Mexico. Brazil pegmatites, some K-feldspar (potassium-feldspar) crystals, roughly 3 m long have been reported. Brownish transparent crystals around 5 cm have been reported from Greenland.
Opaque to translucent, rarely transparent
• Lustre: Vitreous but may be pearly on cleavage surfaces.
• Colour: Commonly colourless and champagne to yellow but also pink, orange, light blue, light green, brown and grey. Massive orthoclase is generally white or pale pink in colour. Predominantly, opaque green orthoclase (monoclinic) with minor ruby inclusions from the Luc Yen area, Vietnam, has been reported. The colour is attributed to its high lead content (0.5wt% PbO). Some translucent and transparent pieces have been polished. White plagioclase with ruby inclusions has also been reported from Myanmar.
• Varieties: Adularia – A moderate to low temperature, more completely ordered orthoclase. It is weakly triclinic (formerly regarded as apparently monoclinic) and typically occurs in well developed, usually transparent and colourless to milky white (and often opalescent pseudo-orthorhombic crystals in fissures in crystalline schists, especially in the region of the Swiss Alps (from Adula Mts, St Gotthard massif, Ticino (Tessin), Switzerland; the type locality). Adularia displays pearly internal reflections. It typically has a relatively high content of barium. ‘Rainbow lattice sunstone’ is the trade name given to orthoclase that exhibits iridescence from hexagonal platelets of hematite on intersecting cleavage planes producing a criss-cross network of colour in addition to adularescence. The material, which is exceptionally rare, comes from the Harts Range about 60 to 70 miles NE of Alice Springs in the Northern Territory, Australia.
• Crystal system: Monoclinic
• Refractive index: 1.518–1.529; – 1.522–1.533; -1.522–1.539. Biaxial ve
• Birefringence: Weak; 0.005–0.010
• Pleochroism: Weak
• Density: 2.56 but has been reported in the range 2.55–2.63
• Hardness: Hardness on Mohs’ scale ranges from 6 to 6.5
• Dispersion: Low: 0.012
• Cleavage/fracture: Perfect prismatic cleavage in 2 directions at 90° to each other. Fracture is uneven, somewhat brittle
• Optical effects: chatoyancy, asterism, colour change, ADR: Yellow and colourless cat’s eye gems are known from Myanmar and Sri Lanka. Some Sri Lankan stones exhibit asterism. An undifferentiated dark brown K-feldspar (RI 1.52; SG 2.6) was reported to exhibit both asterism and chatoyancy. Yellow cat’s eye without adularescence has been reported.
• Absorption spectra (400–700 nm): Absorption spectra described for yellow orthoclase from Madagascar but the material is now known to be high to low sanidine
• Fluorescence: SWUV – commonly red or pink but white, whiteish green, pale green, greyish green, blue and orange have all been reported. LWUV – weak blue or cream. Occasionally weak-to-strong reddish orange under LWUV Simulant: An undifferentiated feldspar has been reported to be dyed blue in an attempt to simulate lapis lazuli. The dye could be easily seen in the cleavages and fractures.